It depends upon how many changes are required to your electrical system - something the installers should be able to estimate prior to the outage.
For our 44 panel/4 PowerWall system, power was down for about 6 hours, while they rewired the grid connections for our 3 house breaker panels, adding an extra cable runway box on the outside, added the TEG panel (and ran the grid power through it) and added a generation panel (with breakers for the solar panels & PowerWalls). During this outage, they also added an extra 50A outlet for emergency charging one of our Tesla vehicles during an extended power outage (hurricane). Because of the extensive changes, they had a team of electricians here all day to do as much of the work as possible before disconnecting the power, and then to do the rest of the work during the outage as quickly as they could.
The installers reminded us that this could be the last time our house would be without power...
In anticipation of this extended outage, we ran a long extension cord from our neighbor's house (with their permission) and had power in one of our rooms during the outage - so it wasn't too bad.